Success factors in perfect ceramic firing
Keywords
Metal ceramics, veneering ceramics, ceramic firing, test of the degree of firing
Over the course of time, there are certain tasks that we carry out automatically,
without being aware of it. We develop routines to keep our minds clear during our
daily activities so that we do not need to think about every single step. As long as
the correct result is achieved, there is no reason for us to make any changes.
However, if there are problems, we should take a closer look at our processes
and investigate why problems have occurred.
The aim of this article is to support dental technicians involved in the fabrication
of ceramic veneer restorations in evaluating and mastering important parameters
in ceramic firing. Beginning with the firing process, the goal is to raise awareness
of the individual steps and success factors that influence a successful outcome in
ceramic restoration work, and to provide criteria that allow users to analyze their
own approach1.
The firing process – what actually happens inside the furnace?
The firing process is comprised of four steps, which are summarized and
illustrated using a heating microscope in Fig. 1:
1. The test sample made of veneering ceramic (picture A) is initially cylindrical. It
then begins to shrink, at a temperature that varies depending on the ceramic.
2. The actual sintering process begins (picture B). As the temperature rises, the
initially square, sharp-edged contours shown in the silhouette become more
and more rounded.
3. Bridges then develop that link to adjacent particles of the powder, and the
ceramic particles "bake" more closely together. During this preliminary phase
prior to complete fusing, the volume of the ceramic shrinks by approx. 10 %
to 12 % (picture C).
4. As the temperature rises, the contours increasingly blend together, and the
sintering process slowly develops into a melting process until the silhouette of
the test sample takes on the shape of a hemisphere (picture D); the melting
point is reached.
The specified maximum firing temperature is, however, considerably lower than
this melting point. Accordingly, the fully-fired dentin material also contains areas
that are not completely fused. Transparency and shade intensity depend on the
proportion of glass-phase matter within the structure. Both become more
intensive as the proportion of glass-phase matter increases.
Fig. 1 Heating microscope images for the determination of temperature-related deformation
behavior during the individual sintering steps for veneering ceramics: (A) Initial shape of the
cylindrical test sample comprised of veneering ceramic; (B) The sintering process begins, contours
that are initially sharp-edged become round; (C) The contours increasingly blend together; (D) The
sintering process slowly develops into a melting process until the silhouette of the test sample takes
on the shape of a hemisphere.
Our eyes only notice the transparency and shade intensity. These differences are
often only attributed to the final firing temperature. However, other factors have
an impact on the ceramic. The degree of firing of a dental ceramic depends, in
addition to this final firing temperature, on other parameters that are described
below.
Exact adherence to the pre-drying time
Urgent deadlines are a common occurrence in dental laboratories. However,
those who take shortcuts when it comes to the pre-drying time are automatically
accepting flaws in the final outcome, as this approach has a crucial impact on the
resulting ceramic (Fig. 2). This is because the surface of the ceramic is already
fused, and liquid and air within can no longer escape. Air bubbles as well as
1
Further details are available in: Michael J. Tholey, Norbert Thiel: Firing dental veneering ceramics.
Quintessenz Zahntech 2009;35(8):1018-1029.
2/8
modeling liquid residue remain within the material as a result. Longer pre-drying
is a very simple way of solving this problem.
Fig. 2 Clear differences in terms of shade due to different pre-drying times with the same ceramic
material: A: Without pre-drying, B: 2 minutes, C: 4 minutes, D: 6 minutes.
The correct lift position
If a restoration is not exposed to sufficient heat during pre-drying, so that the air
and modeling liquid between the powder particles can escape, the ceramic will be
too damp when the heating process begins in the firing chamber. Very small air
bubbles and residual modeling liquid remain inside the ceramic. Due to the
increased refraction of light near the air bubbles, this can cause the veneer to
become cloudy (Fig. 3). This is why it is important to adhere exactly to the
specified lift positions.
Fig. 3 Effect on transparency and shade effect when a crystal-clear material is not fully fired as a
result of different lift positions (A: Furnace 25 % closed, B: Furnace 75 % closed)
Heating time and firing temperature
The degree of firing depends on the temperature and heating time. When heated
slowly, the air in the ceramic escapes more easily. When heated too quickly, it is
more difficult for the air present between the particles to escape, and the ceramic
becomes cloudy (Fig. 4).
3/8
Fig. 4 Ceramic samples (Window material) with different degrees of firing depending on the heating
time (x-axis) and temperature (y-axis)
The optimum hold time
Depending on the ceramic material, the optimum hold time is between one and
two minutes; afterwards, the ceramic can become excessively sintered and may
even fuse despite the relatively low temperature (Fig. 5). A restoration is perfectly
fired when it has a slight shine and edges are visible. If the ceramic has been
over-fired, then the edges are clearly rounded. An excess of glass-phase matter
also has a negative impact on a number of physical properties.
Fig. 5 Schematic view of the fired firing plates
Allowing the ceramic to cool slowly
The cooling time affects the coefficient of thermal expansion (CTE) as well as the
thermal stress remaining within the ceramic. With dental ceramic samples that
have been cooled quickly, a slightly lower CTE is measured than in those that
have been cooled slowly. Slow cooling after firing relieves high levels of thermal
stress within the veneering ceramic.
4/8
The view without a vacuum is clouded
Even if the firing parameters are otherwise identical, ceramic veneers that have
been fired without a vacuum are considerably less transparent than those fired
under vacuum conditions (Fig. 6). Without a vacuum, the ceramic remains milky
and cloudy in appearance, as the air is no longer drawn out of the structure as is
the case under vacuum. The reason for the cloudy appearance is the same as
described earlier.
Fig. 6 Influence of the vacuum on the restoration; with a vacuum (see above), without a vacuum
(see below).
Additives in the modeling liquid
All modern modeling liquids contain additives that improve the modeling
properties, but which can significantly change the shade and transparency of the
ceramic if the pre-drying interval is too short or if it is heated too quickly, or if the
final temperature is too low (Fig. 7). It is therefore important that the liquid can
escape completely. This allows the pre-drying time to be modified if a different
modeling liquid is used.
Fig. 7 Influence of different modeling liquids.
5/8
The framework – a smaller tooth shape for a defined layer thickness
The importance of the framework in a durable ceramic restoration where the
shade does not fade is often underestimated. Only the ceramic-supporting design
of the crown or bridge framework as a miniature version of the final restoration
ensures an even ceramic layer thickness of no more than 1.5 – 2 mm
The size, dimensions and type of framework material are also factors in a good
firing result. Accordingly, a larger 5-unit framework, for example, takes longer
than a single crown to achieve the same level of heat penetration. In this case,
extending the rate of climb somewhat during heating is recommended.
Testing the degree of firing provides for peace of mind
A particular phenomenon is regularly of concern to users: despite identical firing
parameters, the firing results differ in terms of shade. With different types of
furnace, this might be as a result of differences in design. In furnaces of the same
model, this might be due to firing elements that differ in terms of age, for
example, or as a result of different production lots.
The most effective way for the user to get to the bottom of observations of this
kind is by testing the degree of firing2,3. This helps dental technicians to
determine the optimum degree of firing in each of their ovens by themselves. An
evaluation based on surface shine alone is not sufficient as it does not provide
any indication of the degree of firing within the ceramic. The type of test
described here is simple and can be easily reproduced in any dental laboratory.
Step 1: Cleaning firing in accordance with the specifications of the furnace
manufacturer in order to reduce contamination as a result of different alloy
components or the unwanted effects of contamination on the veneered
restoration.
2
Claus H. Ceramic dental materials prior to, during and following the firing process. ZWR
1985;94:612-616.
3
Claus H. A simple test for verification of the grade of firing of the dental ceramic. Dent Lab
1997;45:245-248.
6/8
Step 2: A small amount of crystal-clear or transparent material is applied to a
crown. In order to evaluate rounding, edges should also be modeled.
The temperature and heating rate of the furnace are correct when the firing
sample is transparent, shiny and has sharp edges when it is removed from the
furnace. If the final temperature is too high, the sample will have an "oily" shine,
and the edges will be rounded (Fig. 8).
Fig. 8 Comparison of an incorrectly fired (left: underfired, poor vacuum) and a perfectly fired crown
(right) with VITA VM 13 Window material.
If the final temperature is too low, or if the sample is heated too quickly, it will
have a milky and cloudy appearance. The best way to approach a perfect result
is to use 5 to 10°C increments. A new test sample must be modeled and fired
each time firing is carried out, as once air is trapped inside the veneering
ceramic, it can no longer be burnt out.
Summary
The relationship between firing ceramic dental veneers and the influence of the
various firing parameters on the firing result is complex. The quality of the firing
results can be influenced by changes in the parameters described, as long as the
user follows a systematic and targeted approach when it comes to identifying and
regulating these variables. If the firing result is still not as expected, despite
following the processing instructions provided by the furnace and ceramic
manufacturer, testing the degree of firing is the right starting point for identifying
the reason.
7/8
First published: ZTM 18, 5, 260-263 (2014), Spitta Verlag GmbH & Co. KG,
Balingen, Germany.
Author
Dr. Michael J. Tholey
VITA Zahnfabrik H. Rauter GmbH & Co. KG
Spitalgasse 3, 79713 Bad Säckingen, Germany
8/8